That's something you could totally do I suppose. I think that would lie with the homeowner over you, but if it's a service you want to offer, I say go for it.
I had a few buyers interested in a home before I finished it. I put it on the market while I was still doing updates and repairs in case someone wanted to buy it off me before i had finished to do their own final touches. A couple had requested that if I sold to them, that I would build a deck off the back of the house and install and above-ground pool alongside it. They paid for the wood and pool, I did the manual labor of it and they tacked on an additional 7 grand to the home when they bought it.
One of the sellers I was working with wanted to put up a deck that wrapped around part of the home. He never finished it though. The buyer that ended up getting the home agreed to buy it so long as he pays the cost to get it finished. Porches and decks are great and most people want them.
I never have. I can see how/why someone would but I am just not that handy. I would end up needing to outsource for something like this and it isn't something I want to pay out of pocket for.
You aren't guaranteed to get that money back for the home seller though. Say the home is going to go on the market for $210,000 and you add a porch that cost $12,000, this brings up the minimum asking price to $222,000. If you are only able to make say, $190,000 on the house, it is an additional loss.